Steven Vanackere, Belgian's respected foreign minister, has previously been regarded as a strong ally and staunch supporter of Lady Ashton and her new EU diplomatic service.

But on Wednesday Mr Vanackere went public with his "impatience" of the EU's "high representative" for foreign affairs, who is the best paid female politician in the world.

While accepting that Lady Ashton "cannot be everywhere at the same time" in response to the pace and pressure of world events, Belgium's foreign minister nevertheless questioned her personal track record.

"We can accept that some react faster than Ashton, but with the condition that she can prove that she is working for the medium-term and long-term on very important issues like energy, for example. But I have not seen this either," he told Le Soir newspaper.

The British peer, who has never been elected to public office, has been criticised behind-the-scenes for failing to attend important meetings and for her lack of political or diplomatic successes since taking the job, created by the Lisbon Treaty, in November 2009.

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A growing number of countries, including France, are angry that Lady Ashton's political failure has meant that her newly created European External Action Service (EEAS) has not helped the EU "to speak with one voice," an objective she set herself when taking the job.

During bad-tempered EU summits in February and March, both David Cameron and Nicolas Sarkozy expressed irritation over the EU's foreign minister's grasp on policy over Egypt and Libya. Mr Vanackere lamented deep divisions that emerged within the 27-nation EU during the "great test presented by the Arab awakening".

"We have always wanted the External Action Service to be the central axis around which member states might organise," he said. "But in the absence of a central player that reacts, makes analyses and conclusions quickly, it is the Germans today, the French tomorrow or the English who take up this role. The result is centrifugal, not centripetal."

Lady Ashton's spokesman "refuted" the criticism. "I think we have been up there and have prioritised correctly," he said.

Charles Tannock MEP, the Conservative foreign affairs spokesman in the European Parliament, said the criticism raised questions over the future of Lady Ashton and her EEAS, which has an annual budget of over £404 million.

"MEPs will have to ask some very difficult questions regarding the five month old EU diplomatic service if it does not step up to its envisaged role," he said. "If not we are heading for an existential crisis."

Nigel Farage MEP, the leader of Ukip, described Lady Ashton's "incompetence" as being a major obstacle to the EU's attempts to develop European foreign policy at the expense of national sovereignty.